(a) Field of the Invention
The present invention relates to an inhibitor of the reduction of life cycle of a redox shuttle additive having an overcharge-preventing effect, and a non-aqueous electrolyte and secondary battery comprising the same.
(b) Description of the Related Art
These days, as a reduction in the size and weight of electronic systems is realized and the use of portable electronic devices is generalized, studies on secondary batteries having high energy density are being actively conducted.
A lithium secondary battery, which is currently used, is obtained by using materials capable of lithium ion intercalation/deintercalation as a cathode and an anode, and by injecting a non-aqueous electrolyte between the cathode and the anode. Such a lithium secondary battery generates electric energy via oxidation and reduction reactions induced by the lithium ion intercalation/deintercalation at the cathode and the anode.
However, such lithium secondary batteries have safety problems such as firing and explosion, which result from the use of the non-aqueous electrolyte, and such problems become more severe with an increase in the capacity density of the batteries. Specifically, if the batteries are overcharged to a voltage exceeding the conventional operational voltage, the cathode release an excess amount of lithium, which then produce dendrites on the anode to make the cathode and the anode thermally unstable, thus causing rapid exothermic reactions, including decomposition of the electrolyte. Due to such exothermic reactions, firing and bursting phenomena caused by thermal runaway occur in the batteries, thus causing a problem in the safety of the batteries.
Prior technologies suggested to solve the problem caused by the overcharge of the lithium secondary batteries are broadly classified into methods that use electronic circuits, and methods that use chemical reactions.
The methods that use electronic circuits are methods of mechanically interrupting an electric current by promoting the generation of gas when overcharge takes place. These methods have problems in that much cost is incurred and various limitations arise in processes for designing and fabricating the batteries.
The methods that use chemical reactions include a method to shutdown by fusing the separator, and a method in which an appropriate redox shuttle additive, i.e., an oxidation-reduction reagent that undergoes oxidation-reduction cycling, is added to the electrolyte.
The redox shuttle additive serves to raise the temperature within the battery in an early stage using heat generated from oxidative exothermic reactions so as to shutdown the pores of the separator, thus inhibiting overcharge reactions. Also, the redox shuttle additive protects a battery by consuming an overcharged electric current on the cathode surface upon overcharge and enables the battery to be reused out of the overcharge state.
In the methods using such a redox shuttle additive, the use of lithium halide salts was first attempted, but the volatibility and reactivity of halogen in the form of oxide were pointed out as problems. U.S. Pat. No. 4,857,423 discloses a method using a ferrocene compound, but the method has disadvantages of the low oxidation potential, slow diffusion and low solubility of ferrocene.
Japanese Laid-Open Patent No. Hei 7-302614 discloses, as a redox shuttle additive, a compound comprising an alkyl group or methoxy group as an electron donor linked to a benzene ring. However, in this case, there is a problem in that the effect of the compound is insignificant at high current intensity, because the amount of an electric current capable of undergoing oxidation-reduction cycling is low. U.S. Pat. No. 5,709,968 discloses a compound comprising a halogen compound linked to said benzene compound, but this compound has a shortcoming in that it has a short lifespan when it is used as a redox shuttle additive, because it is highly reactive.